FIG. 1 shows an example of a conventional variable capacity turbocharger to which the invention may be applied. The turbocharger comprises turbine and compressor housings 1 and 2 integrally assembled through a bearing housing 3 by connecting bolts 3a and 3b. A turbine impeller 4 in the turbine housing 1 is connected to a compressor impeller 5 in the compressor housing 2 by a turbine shaft 7 rotatably supported in the bearing housing 3 by a bearing 6. As best shown in FIG. 2 showing section A in FIG. 1 in enlarged scale, the bearing housing 3 is provided, at its turbine housing side, with a shroud 10 comprising plates 9a and 9b between which a plurality of vanes 9 are annularly arranged for guiding into the turbine impeller 4 fluid (exhaust gas) to be guided to a scroll passage 8 of the turbine housing 1, the shroud being sandwiched by the turbine and bearing housings 1 and 3 and secured by the bolt 3a. In FIG. 1, reference numeral 11 denotes a positioning pin for assembly of the shroud 10; and 12, a scroll passage in the compressor housing 2. Reference numerals 13a, 13b, 13c and 13d designate a linked transmission mechanism for control of opening angle of the vanes 9.
The turbine housing 1 formed with the scroll passage 8 has a portion 14 confronting the shroud 10, an annular gap 15 being formed between the shroud 10 and the shroud-confronting portion 14 and extending radially of the turbine shaft 7 into the scroll passage 8. The turbine-housing-side plate 9a constituting the shroud 10 has an extension 17 extending along the turbine impeller 4 toward a notch 16 on an inner periphery of the shroud-confronting portion 14. Thus, the gap 15 extends between the extension 17 and the notch 16 in a direction away from the bearing casing to provide a gap 15′ opening into the inner periphery of the shroud-confronting portion 14.
In FIG. 1, the shroud 10 is provided, at its bearing housing 3 side, with a heat shield plate 18 which is arranged backward of the turbine impeller 4 and is fixed to the plate 9b of the shroud 10. Further, the bearing housing 3 is formed with a portion 19 confronting the heat shield plate 18, a gap 20 being provided between the heat shield plate 18 and the heat-shield-plate-confronting portion 19 and extending radially of the turbine shaft 7.
By nature, the gaps 15 and 20 are unwanted; however, they are provided for countermeasure to, for example, possible thermal deformation of the turbine housing 1 between during being hot and during being cold and possible accuracy dispersion of parts to be assembled.
However, the gaps 15 and 20 may disadvantageously cause gas leakage therethrough from high to low pressure sides, leading to problems such as greatly varied performance at lower pressure side of the turbocharger and resultant unstable engine performance.
In order to overcome the problems, it has been proposed to arrange sealing piston rings in the gap 15′ between the inner peripheral notch 16 on the shroud-confronting portion 14 and the extension 17 of the shroud 10 so as to prevent the gas leakage and absorb thermal deformation (see Reference 1).    [Reference 1] JP 2006-125588A